1. Field of the Invention
The present invention relates to a coordinate input device, and more particularly a device for detecting the Lamb wave entered from a vibration source into a vibration transmitting plate by means of plural sensors provided on the vibration transmitting plate, and measuring the distances from the vibration source and the sensors based on the transmission time of the Lamb wave entered by the vibration source into said vibration transmitting plate.
2. Related Background Art
There is already known a method of calculating the distance between two points utilizing ultrasonic wave, and, as a specific application of such method, there is also known a coordinate input apparatus as disclosed in the U.S. Pat. No. 4,931,965. Such apparatus is composed of a vibration transmitting plate capable of transmitting the vibration and constituting a coordinate input plane, a coordinate input pen capable of generating vibration and constituting a coordinate input instrument, plural sensors mounted on the vibration transmitting plate for detecting the vibration, and a circuit for signal processing and coordinate calculation.
The apparatus is used for outputting the coordinate of the position designated by the coordinate input pen, to an information processing apparatus such as a personal computer. The algorithm for calculating the distance in the apparatus is constructed such that the distances from the vibration source to the sensors are at first determined on the basis of the arrival delay time of the vibration wave and the velocity thereof, and then the coordinate position is calculated in geometrical manner. The above-mentioned conventional apparatus utilizes a Lamb wave as the wave transmitting in the vibration transmitting plate, and the distance L between the vibration source and the sensor is determined by the following equation: EQU L=Vp.multidot.Tp+N.multidot..lambda.p
wherein:
Vp: phase velocity PA1 Tp: phase delay time PA1 .lambda.p: wavelength PA1 Vg: group velocity time PA1 Tg: group delay time
and an integer N is determined by: EQU N=INT{(Vg.multidot.Tg-Vp.multidot.Tp)/.lambda.p+0.5}
wherein:
which will be explained in detail in the embodiment later.
The precision of distance measurement depends on the accuracy of detection of the phase delay time Tp and the group delay time Tg, but it also depends on the preciseness of the constants employed in the calculation, i.e. group velocity Vg, phase velocity Vp and wavelength .lambda.p.
The velocity of Lamb wave is known to be generally dependent on the plate thickness d and the frequency f of the Lamb wave. In the mass production of the distance measuring device utilizing the Lamb wave, certain fluctuation is inevitable in the components constituting the device (for example it is impossible to produce the vibration generating pens of identical vibration characteristics because a difference in the frequency characteristics is unavoidable among the vibration generating elements), so that it is difficult to maintain the frequency f and the plate thickness d constant. Stated differently it is difficult to produce the devices of which a wave velocity is constant, or the production of devices of a constant wave velocity increases the production cost because a tolerance for the components becomes more severe, a production yield is lowered or load for component inspection is increased.
It is therefore conceivable to utilize a method of determining the wave velocity in each device to set the constants for coordinate calculation. The velocity of the vibration wave can be generally obtained by determining the relation between the distance L from the vibration source to the sensor and the time t required for the wave to arrive. However, the measurement for determining such relationship between the distance L and the time t requires considerable time, and the mass producibility is affected if such measurement has to be conducted on each individual device.
For this reason, in order to determine the velocity of the Lamb wave in a simpler manner, there has been proposed a method of measuring the phase period of the detected signal waveform and setting the velocity of the wave based on the period. This method, based on a fact that the velocity (group velocity Vg and phase velocity Vp) of the Lamb wave is dependent on product of the frequency f and the plate thickness d, consists of at first measuring the frequency and then determining the group velocity Vg and the phase velocity Vp of the Lamb wave from thus measured frequency and the already known plate thickness d, which can be easily and promptly measured in the mass production for example with a laser measurement.
Such conventional method has, however, been associated with the following drawback.
When the frequency is derived from the phase period of the detected signal waveform, an error in measurement inevitably is generated, so that the set frequency always involves an error with respect to the actually needed frequency f. Accordingly, the group velocity Vg and the phase velocity Vp of the Lamb wave are determined from the frequency and the plate thickness d, thus becoming different from the actual values. For this reason, there arises a problem that the precision of the distance measurement is lowered.